1. Field of the Invention
This invention relates to a catalyst composition and method for the oxidation of oxidizeable components of a gas-borne stream, e.g., for the treatment of diesel engine exhaust, and more specifically to the treatment of such diesel exhaust to reduce the particulates content thereof.
2. Background and Related Art
As is well-known, gas-borne streams or engine exhausts often contain oxidizeable pollutants such as unburned fuel and vaporized or condensed oils. For example, diesel engine exhaust contains not only gaseous pollutants such as carbon monoxide ("CO") and unburned hydrocarbons ("HC"), but also soot particles which, as described in more detail below, comprise both a dry carbonaceous fraction and a hydrocarbon liquid which is sometimes referred to as a volatile organic fraction ("VOF"), which terminology will be used herein, or a soluble organic fraction. Accordingly, although sometimes loosely referred to as an "exhaust gas", the exhaust of a diesel engine is actually a heterogeneous material, comprising gaseous, liquid and solid components. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide and the oxidation catalyst catalyzes the SO.sub.2 to SO.sub.3 ("sulfates") with subsequent formation of sulfuric acid. The sulfates also react with activated alumina supports to form aluminum sulfates, which render activated alumina-containing catalysts inactive. In this regard, see U.S. Pat. No. 4,171,289 at column 1, line 39 et seq. Previous attempts to deal with the sulfation problem include the incorporation of large amounts of sulfate-resistant materials such as vanadium oxide into the support coating, or the use of alternative support materials such as .alpha.-alumina, silica and titania, which are sulfation-resistant materials. Further, as is known, the oxidation of SO.sub.2 to SO.sub.3 also adds to the particulates in the exhaust by forming condensible sulfur compounds, such as sulfuric acid, which condense upon, and thereby add to, the mass of particulates.
Generally, the prior art has attempted to deal with these problems by dispersing a suitable oxidation catalyst metal, such as one or more platinum group metals, upon a refractory metal oxide support which is resistant to sulfation.
Examples of catalysts designed for the treatment of diesel exhaust fumes and soot include U.S. Pat. No. 4,849,399 to Joy et al dated Jul. 18, 1989. This patent discloses catalytic composites which incorporate sulfur-resistant refractory inorganic oxides selected from the group consisting of titania, zirconia, and alumina treated with titania and/or zirconia (see column 6, lines 62-68).
U.S. Pat. No. 4,759,918 to Homeier et al dated Jul. 26, 1988 discloses catalysts for the treatment of diesel exhaust fumes and soot which incorporate sulfur-resistant refractory inorganic oxides selected from a group which includes silica, alumina, and silica-alumina (see column 3, lines 16-27).